1. Field of the Invention
The present invention relates to: a film formation method for a film used for an element substrate of a liquid discharge head and a substrate on which semiconductor devices, electronics devices, or the like are mounted; a substrate having a base formed with the film, and a liquid discharge head.
2. Related Background Art
A vacuum evaporation method, sputtering method, and the like have been generally used as methods of forming a thin film. However, there is a problem in that, although depending on a film formation material, a metal thin film, which is particularly made of a material that is chemically stable and excellent in corrosion resistance, for example, a noble metal, is apt to be peeled off because the-film has a weak adhesive force with respect to an opposing material, which forms a film formation base, such as SiN, Si, or SiO.
Thus, in order to raise an adhesion strength of the film, there have been introduced physical improvements, which aim at anchor effects, such as optimization of pretreatment for a base, usage of a contact metal, improvement of diffusion which is based on film formation conditions such as a heating temperature, reduction in stress, and increase of surface roughness.
However, in actuality, the improvements are not sufficient in many cases, and peeling is likely to occur from an edge of a pattern opening. Thus, the further improvements for the adhesion strength have been required.
FIG. 5A shows a method in which a pattern opening is formed on a metal thin film 101 on a base 100 through conventional wet etching. A resist 102 is applied onto the metal thin film 101 formed on the base 100, and the resultant is immersed into an etchant from the patterned resist opening to etch the metal thin film 101, thereby forming the pattern opening. Wet etching is isotropic etching. Therefore, an end surface 101a of the pattern opening of the metal thin film 101 corresponds to a surface formed by joining points equidistant from an edge 102a of the resist opening.
FIG. 5B illustrates the case where patterning is performed by conventional dry etching. A metal thin film 111 is formed on the entire surface of a base 110 by sputtering or the like, a photoresist pattern 112 is formed thereon, and then, a pattern opening is formed by reactive ion sputtering (RIE) or sputtering etching with an Ar gas. In the reactive ion sputtering, a reaction gas (chlorine gas, carbonyl chloride, or the like), which combines with a metal as an etching object, is introduced to perform etching through reaction. Thus, an end surface 111a of the pattern opening of the metal thin film 111 can be gentle to some extent in comparison with FIG. 5A. However, a metal material for the film and a selective ratio (an etching rate of the metal thin film 111/an etching rate of the base 110) are limited, and thus, even the base 110 is slightly subjected to etching, which produces a recessed portion 110a. This tendency is conspicuous particularly in the case of a noble metal that is hard to have its selective ratio.
Further, as shown in FIG. 6A, in the case where an adhesion layer 121 is interposed between the base 110 and the metal thin film 111, the adhesion layer 121 is exposed at the end surface 111a of the pattern opening in the above-mentioned dry etching. A material with high reactivity, for example, Ti, Cr, Ni, or Ta is used for the adhesion layer 121. Therefore, the adhesion layer 121 may be dissolved into a kind of agent from the exposed portion, or may cause reaction such as oxidation, and the resultant permeates the inside, which leads to damage of the adhesion layer 121.
FIG. 6B shows a metal thin film 131 formed by patterning through conventional mask film formation. The mask film formation is performed in the state in which a mask 132 with a limited thickness, which is opened at its portion for film formation, is adhered to a base 130. An amount of sputtered particles that fly through sputtering is determined in accordance with the so-called cosine law in which the amount is proportional to cos θ with respect to an angle θ at which the particles are emitted. The amount is maximum at an incident angle of 0°, and is gradually decreased along with the increase of the angle. Therefore, in the case where a side surface of the mask 132 is vertical to the base, a thickness is gradually decreased in an end portion 131a of the metal thin film 131. However, half of the maximum thickness is left at an edge of the film because the edge contacts with the side surface of the mask 132.
Further, patterning through lift-off is known as a modified example of mask film formation. On one hand, in mask film formation, detachment/attachment of a mask is carried out mechanically; on the other hand, in lift-off, a sacrifice layer pattern is provided on a base, a metal thin film is formed thereon, and then, the sacrifice layer pattern is dissolved. In general, the sacrifice layer pattern is formed of a photoresist, and the metal thin film is formed of a desired material to be thinner than the sacrifice layer. Then, a step between the sacrifice layer pattern and the metal thin film is utilized, thereby etching the sacrifice layer from a gap therebetween. As a result, the sacrifice layer pattern and the metal thin film attached thereon are removed together.
However, in the case where the metal thin film made of, for example, a noble metal, is formed, the film formation temperature is high, and thus, a resin resist cannot be used for the sacrifice layer. Further, there has been pointed out, for example, a problem in that the sputtered particles partially destroy the resist pattern, which degrades pattern precision. In order to solve this problem, there is proposed, in Japanese Patent Application Laid-Open No. H07-273280, lift-off with the use of an inorganic sacrifice layer pattern made of Al that is a metal easy to etch.
However, according to the method disclosed in Japanese Patent Application Laid-Open No. H07-273280, the Al sacrifice layer pattern has a rectangular sectional shape, and has a side surface vertical to a base. Thus, when being formed thereon, a metal thin film has half of the maximum thickness at its film edge as in the mask film formation. Further, sputtered particles attach to the side surface of the Al sacrifice layer pattern. Thus, even if an etchant permeates from a thin part of the metal thin film to dissolve Al, the metal thin film is not sufficiently cut between the base and an upper surface of the sacrifice layer pattern. Accordingly, an unnecessary part of the metal thin film needs to be forcedly peeled off.
Further, any conventional element substrate of a liquid discharge head, which ejects a liquid such as ink through utilization of heat energy and which is mounted on a recording device, has a heating resistor (heater) having a heat effect portion. A protective film for the heating resistor needs to have: insulating property that insulates, for example, ink from the heating resistor; corrosion resistance against ink at high temperature; and cavitation resistance against impact of cavitation at the time of disappearance of bubble. Thus, it is general that the heating resistor is protected against a usage environment by means of a protective layer having a two-layer structure in which: an insulating protective film, which is made of SiO2, SiC, SiN, or the like, is formed on the heating resistor; and a protective film with cavitation resistance/corrosion resistance, which is made of Ta or the like, is formed thereon.
It is disclosed in, for example, Japanese Patent Application Laid-Open No. H05-254122 that: a noble metal such as Ir (iridium), Pt (platinum), or Ru (ruthenium) is adopted for the protective film as a material with high corrosion resistance; and this realizes a mechanical strength equal to or larger than that in the conventional one made of Ta (tantalum) and the durability against ink approximately twice or three times the durability in the case of Ta, which achieves twice or three times longer life than the conventional case. However, in the present condition, it is difficult to form a noble-metal thin film into a predetermined pattern, as a result of which manufacturing at low cost has not been realized.